So onset of decay, which is the negative damage in a huge area that heals undead, only hits one or two targets even when surrounded. Tested this a number of times and have never hit more than four targets.
Doesn't seem to be any limitations.
Have you confirmed you are in line of sight of all targets? That does matter, second is that it damages living, heals undead.
Another is that it requires targets to be hostile, aka enemies.
If you can confirm these, then I'll know more.
Yes, tested it in the cheese quest while surrounded by rats.
And it works on some and some it doesnt?
What about healing?
The more possible details I get out of this the better chance there is to fix it eventually when I next look at it.
Likewise, as basic bug report instructions say. Examples of where it was tested, against what creatures, what is your character name, etc.
I no longer have the character as he died, but the healing was hit and miss. If the undead in questions was one of the targets it selected it would heal them.